This invention is in the field of seismic prospecting for oil and gas, and is more specifically directed to signal processing techniques in such prospecting.
In recent years, the use of ocean bottom cable technology for performing seismic surveys in marine regions of the earth has become widespread. As is known in the art, ocean bottom cable technology utilizes acoustic detectors that are deployed at fixed locations at or near the ocean bottom. An acoustic source is towed near the ocean surface, and imparts acoustic energy into the water that is reflected from geological strata and interfaces below the ocean bottom, and measured by the acoustic detectors. The measured signals are, as typical in the seismic prospecting field, indicative of the depth and location of the reflecting geological features. Typically, the ocean bottom detectors include both a geophone and a hydrophone, for recording both pressure and velocity information. This dual-sensor approach enables the elimination of ghost and reverberation effects, for example as described in U.S. Pat. No. 5,774,417, issued Jun. 30, 1998 commonly assigned herewith and incorporated herein by this reference.
Ocean bottom cable detectors are often advantageous, as compared to towed detectors, in performing surveys in crowded offshore regions, such as may be encountered near offshore drilling and production platforms (which are often present, of course, near important hydrocarbon reserves). The cost of each pass of the source vessel through the survey region is also relatively low when using ocean bottom detector cables, considering that the source vessel need not tow hydrophone streamers.
Recently, ocean bottom cable surveys have been used to record multicomponent signals, and thus have provided more information regarding the subsurface lithology in marine regions of the earth. As is known in the art, multicomponent receivers record both pressure and shear wave signals, with the recorded shear wave signals including both horizontal and vertical components. While conventional seismic surveys provide information regarding the location and depth of subsurface formations, comparison of the pressure wave signals to the shear wave components provides useful information concerning the lithology of the reflecting surfaces, further improving the usefulness of the survey in locating important formations from an oil and gas prospecting standpoint. Additionally, multicomponent recording also enables the use of techniques for improving the quality of the seismic data obtained in the marine survey. These benefits and uses of multicomponent recording in the ocean bottom cable context are described, by way of example, in Berg, et al., "SUMIC--Multicomponent sea-bottom seismic surveying in the North Sea--Data Interpretation and Application", presented at the 64.sup.th Annual Meeting of the Society of Exploration Geophysicists (1994).
As is known in the art, lithology information is provided, in the multicomponent survey, by the conversion of the incident pressure waves into reflected shear waves by the reflecting formation. This pressure-to-shear wave conversion generally increases (i.e., is more detectable) with the angle of incidence from the vertical, and as such multicomponent acquisition techniques tend to favor the use of long source-receiver offset distances. It has been discovered, however, that horizontal components of pressure-wave reflections that arrive at slant angles at the receiver can contaminate the shear wave signals, especially at long offsets where the horizontal pressure wave components can swamp (in amplitude) the shear wave signal.
By way of further background, separation of pressure (or P) wave and shear (or S) wave components by signal processing techniques is known. An example of such separation is described in Kendall, et al., "Noise analysis, using a multicomponent surface seismic test spread", presented at the 63.sup.rd Annual Meeting of the Society of Exploration Geophysicists (1993). This approach performs multicomponent rotation analysis at individual receiver positions for each in a series of ray emergence angles, until one is found that maximizes the energy for P and S waves simultaneously. As such, the method described in Kendall, et al. is a relatively complex and computationally intensive approach.